1. Field of the Invention
This invention relates to an optical modulating element, particularly a liquid crystal-optical shutter, more particularly to a liquid crystal-optical shutter suitable for time division driving system (dynamic driving system).
2. Description of the Prior Art
A liquid crystal-optical shutter utilizes the electro-optical modulating function of a liquid crystal, and its modulating portions are arranged in an array, irradiated with light and the transmitted light is taken out selectively, whereby light signals corresponding to the electrical image signals are formed, which light signals being then irradiated on an electrophotographic photosensitive member to obtain digital copies.
Such a liquid crystal-optical shutter array includes the following advantages:
1. When used for an electrophotographic printer, the device as a printer can be miniturized; PA1 2. A mechanical driving portion such as a polygon scanner used in LBP (laser beam printer is not required) and the need for severe mechanical precision is small. Such advantages will naturally give rise to possibilities of improvement of reliability, reduction in weight and lower cost. In fact, however, various problems are involved.
FIG. 1 shows an example of the construction of a liquid crystal shutter array which would is easily understood.
As shown in FIG. 1, openings 11 are provided, with other shaded portions being generally applied with masks so that all light may be blocked. A liquid crystal is sealed between the signal electrodes 13 (13a, 13b, 13c, 13d, . . . ) and the common electrode 14 arranged so as to confront the signal electrodes 13. However, such a liquid crystal-optical shutter, in order to be designed as a shutter having a shorter length of A-4 size with a density of the image formed of 10 dot/mm by arranging the openings according to the mode as shown in FIG. 1, requires about 2000 signal electrodes and the number of drivers necessary for driving respective signal electrodes is also 2000.
The number of drivers in a typical IC, when using IC with 50 pins, is 40. Here, cost reduction is limited.
Alternatively, one may also consider dividing the common electrode into plural lines to be placed in matrix correspondence to the signal electrodes, thereby performing shutter opening and closing with time division for respective lines of the common electrode. When such a liquid array is used as the head for electrophotographic copying machine, the line electrode is required to have a length in the longer direction generally of 150 mm or longer, particularly 210 mm or longer so as to be adapted for the A-4 size according to the Japanese Industrial Standard.
However, according to the study by the present inventor, in a liquid crystal shutter array having long line electrodes in a plural number of lines juxtaposed on one sheet of a substrate and also having light-shielding masks having insulating properties arranged between the line electrodes to prevent light from coming through the gaps between the line electrodes, a great electrostatic capacitance is created between the line electrodes, which is found to make the light transmittance during shutter opening as small as a few percent. As a consequence, the contrast between closing and opening of the shutter is small. For this reason, designing of a photosensitive drum or process designing is rendered difficult in the mounting of such a liquid crystal shutter array on a printer head of an electrophotographic copying machine. In particular, a disadvantage has been found that a satisfactory image cannot be formed when a fluorescent lamp of about 30 W is used as the light source.
As another disadvantage, in the time division driving system wherein the common electrode is divided into a plural number of lines, the electrostatic capacitance formed between each line electrode and the signal electrode confronted therewith differs from line to line, thus giving different transmittance at the respective openings of the shutter array to result in digital copies lacking satisfactory quality. This may be considered to be caused for the following reason. That is to say, the portions, other than the openings in the shutter array, are required to be shielded from light, and hence it is generally practiced to have the portions on the common electrode, except for the openings, masked with a metal such as chromium (in this case, the portions between the respective line electrodes are shielded from light with an insulating black coating), with the result that the metal light-shielding mask act as the common electrode to give an electrostatic capacitance between a first common electrode and the signal electrodes which is different in value from that between a second common electrode and the signal electrodes.